1. Field of the Invention
The present invention relates to an adaptive control device which is arranged to remove periodic noises, that is, noises synchronized with an engine such as vibrations or ambient noises brought about inside of a car with an adaptive filter.
2. Description of the Related Art
Recently, emphasis has been placed on an adaptive control device which offers a very flexible control function of machines or the like according to an environment around such machines. In particular, an adaptive filter for cancelling periodic noises is spotlighted, because the noise periodicity may make its arrangement simpler.
Now, the description will be oriented to such a conventional adaptive control device as mentioned above with reference to the drawings. FIG. 1 shows a schematic arrangement of the conventional adaptive control device. In FIG. 1, a numeral 71 denotes a trigger pulse sensor. A numeral 72 denotes an adaptive filter. A numeral 73 denotes a coefficient update unit. A numeral 74 denotes an actuator. A numeral 75 denotes a sensor. A numeral 76 denotes a transfer characteristic compensating filter. A numeral 77 denotes a transfer characteristic converter.
The operation of the adaptive control device arranged as described above will now be discussed below. In FIG. 1, a pulse synchronized with vibration noises from the outside like engine vibrations is sensed by the trigger pulse sensor 71 and is applied to the adaptive filter 72. Such a pulse is allowed to be easily realized. If, for example, a pulse for a rotary machine like an engine is produced, the use of a rotary sensor for generating a pulse at each minute angle of a rotation realizes easy production of such a pulse. In the case of a car, an engine control unit monitors a rotary pulse signal of a cam or a crank and thus may be used for producing the pulse. The adaptive filter 72 outputs a control signal to the actuator 74 so that the actuator 74 may generate vibrations. The vibrations are added to the noise vibrations at a desired control point. The added result, that is, an error is sensed as an electric signal by the sensor 75 and then is applied to the coefficient update unit 73.
The adaptive filter 72 uses the pulse sensed by the trigger pulse sensor 71 as a trigger and sequentially outputs a value of each tap provided therein. The adaptive filter 72 provides the same number of taps as a quotient obtained by dividing a number of pulses outputted per one rotation of an engine sensed by the trigger pulse sensor 71 by the least significant degree of vibrations to be intentionally controlled. For example, assuming that 50 pulses per one rotation of the engine are outputted and the least significant degree of the vibrations to be controlled is primary, 50/1=50 taps are provided in the adaptive filter 72. The adaptive filter 72, therefore, enables derivation of an output constantly synchronized with the vibrations to be controlled.
On the other hand, the coefficient update unit 73 operates to update a tap coefficient of the adaptive filter 72 based on an algorithm such as a synchronous LMS with an error sensed as an electric signal by the sensor 175 and a transfer characteristic compensating signal obtained by the transfer characteristic converter 77. The transfer characteristic converter 77 serves to convert the output of the compensating filer 76 into a frequency of a current trigger pulse signal. In general, an output delay of the actuator 74 and transfer characteristics from the actuator 74 to the sensor 75 are pre-stored in the compensating filter 76. The adaptive filter is updated based on an adaptive filtered X algorithm by referring to the pre-stored values. The update of the adaptive filter is thus done as considering the delays of these components. At this time, the compensating filter 76 needs the transfer characteristic converter 77 for converting the characteristic of the filter 76, because the characteristic of the filter 76 depends on a sampling frequency and this adaptive control device changes its output frequency on the number of engine rotations.
As set forth above, the conventional adaptive control device provides for cancellation of periodic noises synchronized with engine vibrations such as ambient vibrations inside of a car.
The foregoing conventional adaptive control device, however, has a problem in that all the periodic noises are not allowed to be cancelled if aperiodic noises are mingled in the periodic noises to be intentionally cancelled. To overcome this problem, the conventional adaptive control device has an inserted filter for compensating for the actuator. The actuator compensating filter disables to completely compensate for a delay of the actuator. In particular, if aperiodic noises are mingled, the compensating filter cannot achieve substantially complete compensation so that the output phase may be shifted. The shift of the output phase may cause the control device to malfunction. To avoid such malfunction of the control device, it is possible to reduce the amount of one update of the adaptive filter, that is, a gain of the control system. This reduction, however, makes a converging speed slower, which results in bringing about difficulty in corresponding to change of an environment, concretely, change of noise periodicity.